Object contours in depth maps
If you look at the leaves.blend you'll see that most of the leaves are just planes indeed 



In order keep the polygon count as small as possible, they didn't model every leaf. They used an image with an alpha channel as mask on the materials to create the actual leaf shape instead.




To make this:



Look like that:



The depth map is created with the actual geometry of the objects in space, so it will not take into account "faked" sapes.